Future astronauts could hitchhike their way to Mars—without the need for a Vogon Constructor Fleet. According to a new paper, space explorers could reach the red planet by riding along inside asteroids.

Landing a ship on a space rock would solve a key issue facing Mars travelers: how to shield astronauts from galactic cosmic rays, high-energy particles traveling at near light speed that come from outside the solar system. (Related: "Black Holes Belch Universe's Most Energetic Particles.")

Cosmic rays can damage DNA, increasing the risks of cancer and cataracts for space travelers. Current research suggests that the amount of radiation that would bombard an astronaut during a thousand-day, round-trip Mars mission increases his or her risk of cancer by 1 to 19 percent.

Radiation shields used on Earth are often too heavy for use on spacecraft, and they don't necessarily block cosmic rays. Many spacecraft use light, thin aluminum shields, but in the long term, aluminum hit with cosmic rays can produce secondary radiation that can be worse than the original blast.

During NASA's Apollo missions, moon explorers absorbed high doses of cosmic rays, but only for a short time. Today's space station astronauts are protected from the worst of the radiation by Earth itself—our planet's body and magnetic field block two-thirds of incoming cosmic rays.

By contrast, Mars voyagers would be in deep space with no large body to shield them for up to 18 months.

Instead of focusing on building a better shield, engineers should design spaceships that can hop in and out of passing asteroids, argues study author Gregory Matloff, an adjunct professor of physics at the New York City College of Technology.

The asteroid itself could then block cosmic rays during the voyage—astronauts could pull a Millennium Falcon and park their ship in a crater, or they could use on-board mining tools to tunnel into the rock. (Related: "Obama's New Plan for NASA: Why Go to an Asteroid?")

Mars-Bound Asteroids Coming Soon

According to Matloff's calculations, to be published in the March-April 2011 issue of the journal Acta Astronautica, the asteroid "taxi" would need to be about 33 feet (10 meters) wide to provide enough shielding. It would also need to pass close enough to both planets—within a couple million miles—to make the trip feasible.

Already there are five known asteroids that fit the criteria and will pass from Earth to Mars before the year 2100, based on a database of 5,500 near-Earth objects (NEOs), or comets and asteroids whose orbits take them near our planet.

The asteroids 1999YR14 and 2007EE26, for example, will both pass Earth in 2086, and they'll make the journey to Mars in less than a year. The trouble would be getting home: Because of their wide orbits, it'd be five years before either asteroid would swing around Mars as it heads back toward Earth.

Matloff did find a third space rock that will travel from Mars to Earth—but it makes the journey too early, in 2037. For now it seems a space taxi to Mars would be a one-way ride.

However, the number of NEOs has increased since the database was compiled, Matloff said. There are now more than 7,000 known NEOs, so more potential rock taxis could exist.

Ideally, astronauts would divert an asteroid so that it cycles permanently between Earth and Mars on a well-timed orbit. Humans could nudge an asteroid into the desired path using a solar sail or gentle propulsion. (See "Solar Sail Hybrid Launches From Japan.")

Once the asteroid is in a stable orbit, Matloff said, "you'd just jump on it. You could store provisions and spare parts on it and use it for shielding. ... "

Great Future in Plastics

Nasser Barghouty, a project scientist at NASA's Space Radiation Shielding Project, said Matloff's idea works in theory. But he thinks having so many extra launches and landings would prove too risky.

Like an airline passenger with multiple layovers, "I'd need to hop on so many legs [during the journey]," he said. "That adds to the complexity of the mission, which adds more risk."

A simpler answer is to build lightweight shielding out of something other than aluminum.

The International Space Station, for example, uses plastic panels to help protect its inhabitants from the effects of long-term radiation: "Plastic does the trick," Barghouty said.